1. Field of the Invention
The present invention relates to a capacitor using a solid electrolyte, a circuit board with built-in capacitor, and methods of manufacturing the capacitor and the circuit board with built-in capacitor.
2. Description of the Related Art
A capacitor using a solid electrolyte has a structure in which at least a first electrode, a dielectric layer, a solid electrolytic layer and a second electrode are provided in this order. In conventional way for manufacturing such a capacitor, the dielectric layer is formed on the surface of an anode, which turns out to be the first electrode, by using a method such as an anodic oxidation and the solid electrolytic layer is then formed. Thereafter, a carbon paste or a silver paste is applied onto the surface of the solid electrolytic layer, thereby forming a cathode which turns out to be the second electrode as shown in Japanese Patent Application JP-A-01-32621.
As for the solid electrolytic layer, either an inorganic solid electrolyte such as manganese dioxide, or an organic solid electrolyte such as polypyrrole or a TCNQ complex is used. Otherwise, the inorganic solid electrolyte and the organic solid electrolyte are used together. In any cases, the solid electrolytic layer must be provided on the surface of a dielectric layer.
In a capacitor having such a structure, there is a problem of withstand voltage or leakage current, which is largely influenced by a solid electrolytic layer. Therefore, it is desirable for the capacitor to be formed densely with having more than a certain thickness. In this regard, in the case of forming an inorganic solid electrolyte such as manganese dioxide, the sintering step needs to be repetitively carry out for many times. When the sintering step is repeated, however, a dielectric layer might be damaged through the process. For preventing this problem, the dielectric layer is necessarily formed with relatively thick, however, this causes another problem. That is, in order to form an organic solid electrolytic layer, such as polypyrrole, to be thick and dense, an electrolytic polymerization is suitable to form. However, it is difficult to form the electrolytic polymerization film directly on the surface of the dielectric layer. For this reason, a thin chemical polymerization film is formed on the surface of the dielectric layer to endow a conductivity prior to the electrolytic polymerization film being formed.
In the chemical polymerization film, however, the thickness of the film and the resistance of the film are apt to be varied. In this aspect, it causes further problem in that the property of the electrolytic polymerization film is apt to be varied by the influence of the unevenness. Therefore, in the case in which a capacitor is fabricated in a wiring board to constitute a circuit board with built-in capacitor, if the thickness of the chemical polymerization film has such a large unevenness, it would make unevenness in the thickness of the circuit board with built-in capacitor, which would result in a fatal problem.